Method and apparatus for casting staple-containing propellant grains



March 28, 1967 w. G. BROCK ETAL 3,311,678

METHOD AND APPARATUS FOR CASTING STAPLE-CONTAINING PROPELLANT GRAINS Filed Sept. 17, 1965 VACUUM SOURCE 37 Winford G. Brock FIG.

.FIG. 2

United States Patent Ofifice 3,311,678 Patented Mar. 28, 1967 METHOD AND APPARATUS FOR CASTING STAPLE-CONTAINING PROPELLANT GRAINS Winford G. Brock, Athens, and .loseph E. Nix, Guntersville, Ala., assignors to the United States of America as represented by the Secretary of the Army Filed Sept. 17, 1965, Ser.-No. 488,290 6 Claims. ((11. 2643) The invention described herein may be used by or for the Government for governmental purposes without payment of any royalty thereon.

This invention relates to solid propellants and more particularly to a method and apparatus for casting staplecontaining propellant grains.

Metal staples, that is, thin fragments of metal wire, strip of filament, are employed in solid propellant grains to obtain improved burning characteristics. Incorporation of metal staples throughout the grain provides substantially accelerated burning rates by increasing conduction of heat from the burning surface to the interior of the grain. The staples also serve as a source of highenergy fuel in the propellant mixture, the metal employed being aluminum, magnesium, zirconium or other metal or alloy which releases a large amount of energy upon combustion. Staple-containing propellants are particularly useful in rapid-accelerating rocket motors for applications that require minimum time from launch to target.

One of the problems presented in the use of metal staples in propellant grains is orientation of the staples in the desired manner. Orientation of the staples with their longest axis perpendicular to the burning surface would provide maximum effectiveness, but no practical method is available to orient the staples in this manner. Orientation parallel to the burning surface is undesirable in that a minimum increase in burning rates is realized.

Control of staple orientation has proven particularly difficult for propellant grains prepared by slurry casting. In this method the uncured propellant mixture, while in a pasty gel state, is poured under pressure into a mold of the desired configuration, normally an elongated cylindrical container having an axially disposed central mandrel to produce an internal perforation. The internal perforation provides an internal burning surface extending throughout the body of the grain, and the amount of burning surface is controlled by varying the design of the perforation. For example, star-shaped internal perforations are employed to obtain larger burning surfaces. In casting propellant grains of this type, the metal staples tend to become aligned in the direction of flow of the viscous propellant mixture, which is at least partially parallel to the mold and mandrel walls. As a result, the staples are aligned partially parallel to the burning surface inan irregular manner. It may be readily seen that propellant grains having partially parallel, irregularly aligned staples will not burn reproducibly. Random orientation of staples throughout the grain, although not as desirable for acceleration of burning rates as perpendicular orientation, would provide reproducible results while maintaining adequate burning rates.

It is therefore an object of this invention to provide a method of casting staple-containing propellants.

Another object is to provide a method of casting staplecontaining propellant grains wherein the staples are oriented in a substantially random manner.

Another object is to provide apparatus for casting ran dom-oriented, staple-containing propellant grains.

Other objects and advantages of this invention will be apparent from the following detailed description.

. In the present invention staple-containing propellant grains are prepared by intermittently passing an uncured, staple-containing propellant mixture through a plate having circular orifices of a predetermined diameter whereby discrete, generally spherical pellets are formed, disposing the pellets as a bed in a mold degassing and curing the pellet bed whereby the pellets merge into a continuous grain. In this method the staples are aligned parallel within the individual pellets during passage through the orifices, but the pellet bed is formed of a large number of randomly oriented pellets so that orientation of staples in the grain as a whole is substantially random. By use of a perforated plate and intermittent pressurized gas pulsing of the propellant through the plate, large quantities of pellets are readily formed and large molds can he filled rapidly. Random orientation of the pellets is reproducible so that performance of the grain can be predicted with confidence.

The invention is illustrated by the accompanying drawings of which:

FIGURE 1 is a sectional view of a rocket motor casting assembly;

FIGURE 2 is an end view of a perforated plate for use in forming pellets.

Referring to FIGURE 1, there is shown a generally cylindrical casting can 1 provided with a sloping bottom plate 2 and a cover 3. The lower portion of can 1 contains the staple-containing propellant mixture. A movable float 4 rides above the propellant level and transmits pressure downward. A plunger 5 of circular plate configuration and having a pressure seal 6 mating with the inner surface of the wall of the can 1 is disposed in the upper part of the can spaced apart from the float, the region between the float and plunger serving as a gas pressure chamber. A centering shaft 6 extending from plunger 5 through orifice 7 in can cover 3 is provided to maintain alignment of the plunger. The plunger is penetrated by a duct 8 connected to a flexible hose 9 terminating at duct 10 in the can cover, duct 10 being connected to valved pressure source 11. The lunger is also penetrated by a second duct 12 connected to a flexible hose 13 extending to a second duct 14 in the can cover. A variable and fluctuating pressure valve 15 is connected to duct 14. Ducts 16 and 17 connected to casting hoses l8 and 19 are provided at the bottom of the casting can for passage of the propellant mixture. Flow valves 2@ and 21 in the casting hoses serve .to control the flow therethrou-gh. Perforated metallic plates 22 and 23 attached to the ends of the casting hoses by means of clamps 2d and 25 are provided for formation of pellets.

A generally cylindrical casting pit and curing oven 26 is disposed in bay floor 27 below the casting can. Cover 28, secured to the top of the casting pit and curing oven, is penetrated by the casting hoses at ducts 29 and 30. A cylindrical motor case and casting mold 31 is disposed within the casting pit and curing oven, motor case support 32 being located underneath the motor case to hold it in position. Vibrators 33 and 34 are provided in the motor case support to vibrate the motor case. An elongated mandrel 35 is positioned longitudinally through the center of the motor case to form an internal perforation in the resulting grain. Casting hoses 18 and 19 extend downward into the motor case on opposite sides of the mandrel. Cover 28 is penetrated by duct 36 connected to a vacuum source 37 and by duct 38 connected to a variable and fluctuating vacuum Valve 39. A transparent viewport 40 is also provided in cover 28 for visual observation of propellant grain preparation. A conventional hydraulically actuated elevator 41 is provided to support and raise and lower the cast-ing can assembly.

FIGURE 2 shows perforated plates 22 and 23 having a large number of circular orifices 42 extending therethrough for pellet formation.

Randomly oriented staple-containing propellant grains are prepared in the above described apparatus as follows.

A staple-containing propellant mixture is placed in the lower portion of casting can 1, and float 4 is placed on top of the mixture. Cover 3, having attached thereto plunger 5 and its accompanying pressurizing assembly, is then secured to the top of the casting can, plunger 5 being spaced apart from float 4 to form a pressure chamber. A gas such as air or nitrogen is then introduced into the chamber. The pressure in the chamber is intermittently varied by means of valve from a level sufficiently high to force the propellant mix-ture through orifices to a lower level at which movement of the mixture stops. A generally spherical pellet of propellant mixture is formed below each orifice as the mixture emerges after application of the higher pressure level. The resulting pellets fall by gravity and form a random bed in motor case 31, the motor case being vibrated by vibrators 3o, 34 to enhance movement and settling of pellets in the bed.

The motor case assembly is kept under vacuum by means of valve 39 and vacuum source 37 to remove gases from the bed and to assist in forcing the mixture through the orifices. The casting can and attached casting hoses are progressively raised by means of elevator 41 as the level of the bed rises.

When the bed reaches the desired height in the motor case the propellant mixture is cured and consolidated by mild heating for an extended period, the time and temperature depending on the composition of the particular propellant mixture. The pellets merge during curing to form a continuous grain with staples oriented only within the local sites occupied by each discrete pellet before curing. Orientation of the sites with respect to one another is random throughout the grain.

The method of this invention is broadly applicable to staple-containing propellant mixtures which exhibit suitable physical properties in the uncured state for formation of discrete pellets with sufficient rigidity for randomizing movement upon falling into a pellet bed. The viscosity of the mixture should be high enough that no significant flow through the orifices occurs without application of superatmospheric pressure. If the viscosity becomes too high the orifices will become clogged and excessive pressures will be required. In general this method is useful for composite-type propellant mixtures comprising a liquid polymeric binder such as polysulfides, polyurethanes, polydienes, polybutadiene-acrylic acid copolymers and the like; an inorganic oxidizer such as ammonium perchlorate or other chlorate or nitrate salt; and other additives such as curing agents and burning rate catalysts, along with the metal staples. Propellant mixtures of this type normally contain a relatively large amount such as 50 to 80 weight percent of the solid-phase oxidizer so that the uncured mixture is a thick, doughlike formulation having the desired viscosity. Metal staples normally comprises about 1 to 7 weight percent of the propellant mixture.

The chemical composition of the propellant mixture is not critical, and other types of staple-containing propellants such as double-base propellants employing nitrocellulose as the hinder and'a nitrate ester as plasticizer may also be fabricated by this method. If the viscosity of the mixture is too low, partial curing, that is allowing the mixture to partially harden by means of its usual curing reaction, may be employed prior to pellet forma- .tion.

The metal staples employed in propellant mixtures are fragments of wire about /6 to inch long and 5 to microns in diameter or metal strips of the same length, .02 to .06 inch wide, and .004 to .010 inch thick. The size of the staples, however, is not critical and propellant mixtures containing staples outside this range may be fabricated by the present method. It is preferred to use staples of the same size and shape to obtain a uniform burning rate throughout the grain.

The diameter of the orifices through which the mixture is forced is selected to allow passage of the mixture and cause alignment of staples throughout the resulting pellet.

A diameter of A; to /2 inch is preferred. Smaller orifices tend to become clogged, and at a diameter over /2 inch the staples in the center are not uniformly aligned. A multiplicity of uniform-sized, regularly spaced orifices is employed to obtain a large number of pellets falling simultaneously. The orifices are conveniently provided as circular holes in a metal plate. The thickness of the plate is not critical, but about to A is preferred.

Superatmospheric pressure is required to force the mixture through the orifices. The exact pressure will depend upon the particular mixture, higher pressure being required for mixtures having higher viscosities. About to pounds per square inch is suitable for typical compositions. The pressure is applied to the mixture in intermittent pulses to cause the mixture to emerge as generally spherical pellets rather than extruded strands. In each pulse cycle the pressure is increased to the level required to cause flow through the orifices and is then decreased to a lower level, thereupon flow ceases and the resulting pellet drops. The pellets are formed in generally spherical shape, which is intended to include short cylindrical shapes with rounded ends as well as spheres.

The ratio of length to diameter of any cylindrical pellet should be less than two to allow random movement in the bed.

A portion of the force for forming the pellets may be supplied by use of a vacuum below the orifices and over the pellet bed. This measure also serves to remove gases from the pellet bed, degassing of the bed prior to curing being required to obtain a continuous grain.

Although not critical, vibration of the pellet bed by conventional mechanical means is preferred to obtain increased randomizing motion of the pellets and settling of the bed.

Curing of the propellant mixture in the bed is effected by conventional methods, normally by mild application of heat for an extended period. In a preferred procedure the apparatus employed for formation of the pellet bed in the motor case is provided with heating means so that curing can be carried out in place. Alternatively, the motor case may be placed in a separate curing oven for this purpose.

The method of the present invention is not limited to a particular type or size of propellant grain, and the apparatus containing the orifices may be adjusted to provide distribution of pellets on all sides of the motor case. Conventional casting hoses having perforated plates attached to the ends thereof may be employed, with the number of hoses depending on the size and shape of the motor case. For typical perforated grains two to three casting hoses provide a suitable pellet distribution.

Although the method disclosed above is described with reference to a particular apparatus, it is to be understood that various changes and modifications can be made in the apparatus without departing from the scope of the invention, which is limited only as indicated by the appended claims.

What is claimed is:

1. The method of preparing a staple-containing solid propellant grain which comprises intermittently passing a staple-containing propellant mixture in the uncured, dough-like state through a barrier having a plurality of circular orifices of a diameter such as to cause alignment of said staples upon passage therethrough whereby said mixture is formed into a multiplicity of substantially spherical pellet-s, introducing a stream of said pellets into a mold whereby a randomized pellet bed is formed in said mold, degassing said pellet bed and curing the resulting degassed pellet bed.

2. The method of preparing a staple-containing solid propellant grain which comprises intermittently passing a staple-containing propellant mixture in the uncured state and having a viscosity high enough to preclude significant flow except under superatmospheric pressure through a barrier having a plurality of circular orifices from about A; to /2 inch in diameter, said mixture being passed through said barrier for intermittent periods sutficient to produce substantially spherical pellets, dropping the resulting pellets downward into a mold whereby a randomized pellet bed is deposited in said mold, degassing said pellet bed and curing the resulting degassed pellet bed.

3. The method of preparing a staple-containing propellent grain which comprises disposing an uncured, staple-containing propellant mixture having a viscosity such as to preclude significant flow except under superatrnospheric pressure into a container provided with a gas pressure chamber in the upper portion thereof, a propellant-containing region in the lower portion thereof and at least one barrier having a plurality of uniform circular holes about A; to /2 inch in diameter extending therethrough in communication with said propellant-containing region, intermittently increasing the gas pressure in said chamber to a level suchas to force said mixture through said holes for a period of sufiicient duration to form said mixture into substantially spherical pellets, dropping said pellets into a mold disposed below said barrier whereby a randomized pellet bed is deposited in said mold, a vacuum atmosphere being maintained in said mold, and curing the resulting pellet bed.

4. The method of claim 3 wherein said gas pressure is intermittently increased to a level of about to pounds per square inch.

5. The method of claim 3 wherein said pellet bed is subjected to mechanical vibration during deposition.

6. Apparatus for preparing staple-containing solid propellant grains comprising in combination an enclosed generally cylindrical, vertically extending casting can forming a propellant-containing region in its lower portion and a gas pressure chamber in its upper portion, downwardly movable pressure-transmitting means disposed between said regions, means for intermittently introducing pressurized gas into said chamber, at least one barrier at the bottom of said can communicating with said propellantcontaining region, said barrier having a plurality of circular, uniform sized orifices Vs to /2 inch in diameter extending therethrough, a mold disposed below said barrier, and mold degassing means.

No references cited.

L. DEWAYNE RUTLEDGE, Primary Examiner. 

1. THE METHOD OF PREPARING A STAPLE-CONTAINING SOLID PROPELLANT GRAIN WHICH COMPRISES INTERMITTENTLY PASSING A STAPLE-CONTAINING PROPELLANT MIXTURE IN THE UNCURED, DOUGH-LIKE STATE THROUGH A BARRIER HAVING A PLURALITY OF CIRCULAR ORIFICES OF A DIAMETER SUCH AS TO CAUSE ALIGNMENT OF SAID STAPLES UPON PASSAGE THERETHROUGH WHEREBY SAID MIXTURE IS FORMED INTO A MULTIPLICITY OF SUBSTANTIALLY SPHERICAL PELLETS, INTRODUCING A STREAM OF SAID PELLETS INTO A MOLD WHEREBY A RANDOMIZED PELLET BED IS FORMED IN SAID MOLD, DEGASSING SAID PELLET BED AND CURING THE RESULTING DEGASSED PELLET BED.
 6. APPARATUS FOR PREPARING STATPLE-CONTAINING SOLID PROPELLANT GRAINS COMPRISING IN COMBINATION AN ENCLOSED GENERALLY CYLINDRICAL, VERTICALLY EXTENDING CASTING CAN FORMING A PROPELLANT-CONTAINING REGION IN ITS LOWER PORTION AND A GAS PRESSURE CHAMBER IN ITS UPPER PORTION, DOWNWARDLY MOVABLE, PRESSURE-TRANSMITTING MEANS DISPOSED BETWEEN SAID REGIONS, MEANS FOR INTERMITTENTLY INTRODUCING PRESSURIZED GAS INTO SAID CHAMBER, AT LEAST ONE BARRIER AT THE BOTTOM OF SAID CAN COMMUNICATING WITH SAID PROPELLANTCONTAINING REGION, SAID BARRIER HAVING A PLURALITY OF CIRCULAR, UNIFORM SIZED ORIFICES 1/8 TO 1/2 INCH IN DIAMETER EXTENDING THERETHROUGH, A MOLD DISPOSED BELOW SAID BARRIER, AND MOLD DEGASSING MEANS. 